Rubber compositions for a tire tread containing a natural rubber or the like as a main component have been proposed these days from the viewpoints of such issues as depletion of oil resources, reduction in rolling resistance, and environmental concerns. However, in the case that only a natural rubber is used as a rubber component, the wet grip performance of the obtained tread rubber is problematically inferior to that of conventional tread rubbers containing a styrene butadiene rubber as a main component.
In order to solve the problem, attempts have been made to improve the wet grip performance by blending an epoxidized natural rubber as a rubber component. Use of an epoxidized natural rubber as a rubber component provides a good wet grip performance; however, as compared with conventional tread rubbers containing a styrene butadiene rubber as a main component, the crack resistance and abrasion resistance need to be improved in especially the case that the obtained tread rubber is used for high performance tires (very low profile tires) or tires for heavy load vehicles among passenger vehicles. Regarding the crack resistance used herein, it has been desired to prevent cracking at the bottom of the tread groove in the direction of the groove, which is generated particularly by ozone or repeated strains.
In the case that an epoxidized natural rubber, natural rubber, butadiene rubber or the like is vulcanized, reversion may occur. The phenomenon of reversion includes deterioration of the rubber and degradation of the crosslinked structure. Results of studies conducted by the present inventors have revealed that the abrasion resistance and the rolling resistance property are deteriorated due to the phenomenon. Also, rigidity may be decreased, leading to poor handling stability. Moreover, if vulcanization is performed at higher temperatures so as to increase productivity of tires, the foregoing phenomenon may become more significant.
As methods to inhibit reversion of the rubber compositions and improve the heat resistance, techniques such as a method of increasing the amount of a vulcanization accelerator relative to the amount of sulfur as a vulcanizing agent and a method of blending a thiuram vulcanization accelerator as a vulcanization accelerator have been known. Meanwhile, as crosslinking agents capable of forming a long-chain crosslinked structure represented by —(CH2)6—S— or the like, PERKALINK 900 and Duralink HTS (both produced by Flexsys), Vulcuren KA9188 (produced by Bayer AG), and the like have been known. It is known that, by blending such a crosslinking agent in the rubber composition, reversion of the rubber composition can be inhibited. The foregoing methods have an effect on the inhibition of reversion of natural rubber or isoprene rubber; however, they problematically have little effect on the inhibition of reversion of epoxidized natural rubber or butadiene rubber.
Patent Document 1 has disclosed the effect of concomitant use of an epoxidized natural rubber and calcium stearate in improving the rubber strength. Patent Document 2 has disclosed the effect of blending a mixture of a zinc salt of an aliphatic carboxylic acid and a zinc salt of an aromatic carboxylic acid with a rubber component for inhibiting reversion. However, there is still room for improvement in achieving inhibition of reversion and at the same time achieving environmental performance (excellent rolling resistance, response to depletion of oil resources, attention to CO2 emission), wet grip performance, dry grip performance, handling stability, and durability (abrasion resistance, crack resistance) in a well-balanced manner.
Patent Document 1: JP-A 2007-169317
Patent Document 2: JP-A 2007-321041